I am taking levothyroxine 150 µg daily and my labs show suppressed thyroid‑stimulating hormone (TSH) 0.158 µIU/mL with normal free thyroxine (free T4) and total triiodothyronine (total T3); how should I adjust my levothyroxine dose?

Reduce Your Levothyroxine Dose Immediately

Your current levothyroxine 150 µg dose is too high and causing iatrogenic subclinical hyperthyroidism (TSH 0.158 µIU/mL with normal T4 and T3), which significantly increases your risk of atrial fibrillation, bone loss, and cardiovascular complications—you need to reduce your dose by 12.5–25 µg immediately. 1

Why Your Current Dose Is Excessive

Your laboratory results show:

• TSH 0.158 µIU/mL (reference range 0.4–4.5 µIU/mL): This is suppressed below the normal range 1
• Free T4 1.50 ng/dL: Within normal limits 1
• Total T3 74 ng/dL: Within normal limits 1

This pattern—suppressed TSH with normal thyroid hormones—defines iatrogenic subclinical hyperthyroidism, meaning your levothyroxine dose is creating a state of thyroid hormone excess even though your T4 and T3 appear normal. 1, 2

Approximately 25% of patients on levothyroxine are unintentionally maintained on doses sufficient to fully suppress TSH, increasing serious complication risks. 1 You are currently in this group.

Serious Health Risks You Face Right Now

Cardiovascular Complications

• Atrial fibrillation risk increases 3–5 fold when TSH is suppressed below 0.4 µIU/mL, especially if you are over 60 years old 1
• Increased cardiovascular mortality is associated with prolonged TSH suppression 1
• Abnormal cardiac output and potential ventricular hypertrophy develop with long-term TSH suppression 1

Bone Health Risks

• Accelerated bone mineral density loss, particularly if you are postmenopausal 1
• Increased risk of osteoporotic fractures, especially hip and spine fractures in women over 65 years with TSH ≤0.1 µIU/mL (though your TSH of 0.158 carries lower but still elevated risk) 1

Silent Nature of These Risks

The most dangerous aspect is that you likely feel fine—large population studies show no association between low TSH and physical or psychological symptoms of hyperthyroidism, meaning the cardiovascular and bone damage occurs silently. 1

Immediate Dose Adjustment Required

Reduce your levothyroxine dose by 12.5–25 µg (from 150 µg to either 137.5 µg or 125 µg daily). 1, 3

Choosing Between 12.5 µg vs. 25 µg Reduction

Use a 25 µg reduction (to 125 µg daily) if:

• You are under 70 years old without cardiac disease 1
• Your TSH is closer to 0.1 µIU/mL 1

Use a 12.5 µg reduction (to 137.5 µg daily) if:

• You are over 70 years old 1
• You have any cardiac disease, atrial fibrillation, or multiple comorbidities 1
• You prefer a more conservative approach 1

Monitoring After Dose Reduction

• Recheck TSH and free T4 in 6–8 weeks after the dose change, as this represents the time needed to reach a new steady state 1, 3
• Target TSH range: 0.5–4.5 µIU/mL with normal free T4 levels 1, 3
• Once your TSH normalizes, monitor every 6–12 months or sooner if symptoms change 1, 3

Special Considerations

If You Have Thyroid Cancer

If your levothyroxine was prescribed for thyroid cancer requiring TSH suppression, do not reduce your dose without consulting your endocrinologist first, as target TSH levels vary by cancer risk stratification:

• Low-risk patients with excellent response: TSH 0.5–2.0 µIU/mL 1
• Intermediate-to-high risk patients: TSH 0.1–0.5 µIU/mL 1
• Structural incomplete response: TSH <0.1 µIU/mL 1

However, even for most thyroid cancer patients, a TSH of 0.158 µIU/mL represents excessive suppression unless you have high-risk disease. 1

If You Were Prescribed Levothyroxine for Hypothyroidism

Dose reduction is mandatory—there is no therapeutic benefit to TSH suppression in primary hypothyroidism, only harm. 1

Critical Pitfalls to Avoid

• Never ignore suppressed TSH just because you feel fine—the cardiovascular and bone damage occurs silently before symptoms appear 1
• Do not wait to reduce your dose—every month of continued TSH suppression increases your cumulative risk of atrial fibrillation and fractures 1
• Do not adjust your dose more frequently than every 6–8 weeks—levothyroxine requires this interval to reach steady state, and premature adjustments lead to overcorrection 1, 3

Additional Protective Measures

While normalizing your TSH is the most important intervention:

• Ensure adequate calcium intake (1200 mg/day) and vitamin D (1000 units/day) to protect bone health during the period your TSH has been suppressed 1
• Consider an ECG to screen for atrial fibrillation, especially if you are over 60 years or have any cardiac symptoms 1
• If you are a postmenopausal woman with prolonged TSH suppression, discuss bone density assessment with your physician 1